1. Field of the Invention
The present invention relates to a focus detection device. More specifically, the present invention relates to a focus detection device wherein two or more rows of digital (meaning "discrete") sensors of a predetermined pitch are arranged in the focal plane of a photographic lens for forming an image of a subject on a photographic material in such a manner that the respective rows of digital sensors may read substantially the same location of the subject when focusing is achieved, and the respective rows of digital sensors may read different locations of the subject when focusing is not achieved, thereby detecting whether or not focusing has been achieved by the presence or absence of the difference between the readout signals of the respective rows of digital sensors.
2. Description of the Prior Art
Various focus detection devices of the type described above have been proposed. Some typical examples will first be described.
FIG. 1 shows an optical system for detecting whether or not focusing has been achieved by dividing the image into two divided images by a split prism, receiving each image which may be in or out of focus at a set of sensors arranged in a row, and comparing the outputs of the respective rows of sensors. Referring to FIG. 1, an image 4 of a subject 1 is vertically divided by a split prism 3.sub.1, 3.sub.2 disposed in the focal plane of a photographic lens 2. Divided images 4'.sub.1 and 4'.sub.2 of the image 4 are guided through a relay lens 5 to a row of photosensors 6.sub.11, 6.sub.12, 6.sub.13, . . . and another row of photosensors 6.sub.21, 6.sub.22, 6.sub.23, . . . , respectively. Whether focusing has been achieved or not is determined according to whether or not the divided images 4'.sub.1 and 4'.sub.2 (these are the divided images corresponding to substantially the same location of the image, although they differ in vertical location but occupy the same position in the transverse direction) of the image 4 coincide each other.
In this system, parts of the subject differing in vertical location are detected by the two rows of sensors when focusing is achieved and the common part of the subject corresponding to the overlapping of the image is detected by these two rows of sensors. As an improvement over this system, an invention was made according to which the split prism is divided into two stages to vertically divide the image (Laid-open Japanese Patent Application No. 53-143315). FIG. 2 shows a system for detecting (by photoelectric conversion elements) the same part of the subject when focusing is achieved and different parts of the subject when focusing is not achieved. Referring to FIG. 2, rays forming the image 4 of the subject 1 converge at a focal plane F from the photographic lens 2. A pair of convex lenses 7.sub.1 and 7.sub.2 is arranged behind the focal plane F. Photoelectric conversion elements 5.sub.11 and 5.sub.21 are arranged at optically equivalent positions of the images formed by this pair of convex lenses. Whether or not focusing has been achieved is judged by comparing the outputs from these two photoelectric conversion elements (Laid-open Japanese Patent Application No. 54-7323). A plurality of pairs of convex lenses and photoelectric conversion elements may alternatively be incorporated. FIG. 2 shows the condition under which focusing is not achieved.
FIG. 3 shows an improvement over the system shown in FIG. 2.
Referring to FIG. 3, the image 4 of the subject 1 focused is formed by the photographic lens 2 on rows of fly-eye lenses 8.sub.1, 8.sub.2, . . . This image becomes incident on a pair of sensors 5.sub.11 and 5.sub.21 arranged behind the respective fly-eye lenses after being transmitted therethrough. In practice, the same number of pairs of sensors 5.sub.11, 5.sub.12, 5.sub.13, . . . , 5.sub.21, 5.sub.22, 5.sub.23, . . . as the number of fly-eye lenses are used, and the outputs of the respective pairs of sensors are compared to judge whether or not (focal point is deviated to the front or to the back of the depth of field) focusing is achieved (U.S. Pat. No. 4,185,191).
The optical feature common to all three examples described above is that at least one pair of sensors detects the same depth of field irrespective of whether or not the same location of the object space is detected. Another common feature is that, as the sensor arrangement, a CCD (charge-coupled device), a CID (charge injection device) or the like may preferably be adopted.
The present invention relates to an optical low-pass filter which may be applied to all these systems as described above and which has important functions that may even determine the practicality of these systems.
The applicant of the present invention has proposed a focus detection device which operates by detection of overlapping images. The device is different from the respective systems as described above and uses a CCD line sensor. It has been found that, with this proposed system, when the subject moves or the system is moved while the MTF of the optical system for forming an image on the sensor plane is high, the image flickers on the sensor plane. Especially with a subject with high contrast, for example, an edge chart of black and white, the electrical signal processing becomes difficult. A focus detection device which has optical low-pass filter effects for solving this problem is now proposed.
This problem will first be described in more detail referring to FIGS. 4 and 5.
The upper graph of FIG. 4 shows the intensity distribution I(x) of the edge image formed on a series of sensors S.sub.1, S.sub.2, . . . wherein curve I.sub.1 represents an intensity distribution I(t.sub.1) at a time t.sub.1 and curve I.sub.2 represents an intensity distribution I(t.sub.1 +.DELTA.t) at a time t.sub.1 +.DELTA.t. Although the pitch p of the sensors is about 0.05 to 0.2 mm, the image on the sensors is easily displaced by one pitch during a very short period of time due to movement of the subject, movement of the system or the like, and curves I.sub.1 and I.sub.2 overlap.
The lower graph of FIG. 4 shows a video signal V(x). The video signal V(x) abruptly changes from level V.sub.1 to level V.sub.2 for the sensor S.sub.7. Since such abrupt output changes occur for sensors of the other series, the signal processing cannot follow the flickering of the input signals (flickering of the intensity distribution of the sensors) when comparing by complex signal processing the signals from the two series of sensors for judging whether or not (focal point is deviated to the front or to the back of the depth of field) focusing is achieved. This results in erratic operation.
Even if the signal processing follows the flickering, the display lamp, which signals whether or not focusing is achieved, tends to flash.
As shown in FIG. 5, when image data having a finer structure than the sensor pitch is input and when there is a shift in the position (phase) of the image relative to the sensors, different outputs are obtained for the same image. Thus, the two graphs out of four at the right of FIG. 5 show the case when the phases of the image on the sensors are shifted from each other upon reception of the input (image intensity distribution) by the sensors. The two graphs at the left of FIG. 5 show the sensor output values of the respective cases.
When it is assumed that the outputs of adjacent sensors are added and compared or processed, different output values are obtained for the same image data: EQU V=.DELTA.V.sub.1 +.DELTA.V.sub.1 =2.DELTA.V.sub.1
for the upper graphs and EQU V=.DELTA.V.sub.2 +.DELTA.V.sub.2 =2.DELTA.V.sub.2 =.DELTA.V.sub.1
for the lower graphs.
In order to moderate such a difference in the output values, for a high resolution optical system wherein the leading or trailing edge is less than half the sensor pitch as shown in FIG. 4 or an image is formed having a finer structure than the sensor pitch, the resolution may be lowered so that the leading or trailing edge may become about the same as the sensor pitch or greater. An optical low-pass filter is used for lowering the resolution of sensors of high resolution optical systems.
Various types of optical low-pass filters are known, especially the type which are arranged in front of the image pickup tube of a television camera. These include, for example, various types of diffraction gratings of the amplitude or phase type, random dot filters (amplitude or phase type), lenticular lenses, fiber plates, birefringence parallel plates, or the like.